Pages

Wednesday, July 9, 2014

The Proterozoic Eon, Part 4

It's now 2.1 billion years ago, in the Rhyacian Period of the Proterozoic Eon, and the planet is finally thawing out from the 300 million year ice age triggered by those cyanobacterial bastards.

Things are getting back to normal, here in Toronto. I mean, the land is still a barren desert, devoid of life except for those desert crusts we saw earlier, but at least the coasts are regaining a bit of their sparkle. Let’s take a walk along the beach, shall we?

We can see that the algal mats are back, albeit as a cyanobacteria monoculture and not the beautiful, multicolored ecosystems we saw when we first got here. Ah, but what happens when we look at a water droplet under the cartoon microscope?

With the cartoon microscope (which has lower resolution but better clarity than the regular microscope,) we can see that an incredible development has occurred, probably sometime within the past 400 million years. The cell on the right is a prokaryote. These have been around since before we got to the Proterozoic Eon. The cyanobacteria in those algal mats are all prokaryotes.

But the cell on the left is something new. It's a eukaryote, a more complex cell that's larger, with internal structures (called organelles) specialized to particular tasks. You may be tempted to think of them as more evolved than the prokaryotes, but that's your own eukaryotic biases talking. In this time, the eukaryote is nothing but a curious niche organism. Its complexity and size slows down its reproduction cycle, leaving it unable to keep up with the sleeker, more efficiently growing prokaryotes. And all those specialized organelles? No doubt they can be an advantage, but they're also something else that can go wrong. Consequently, these new eukaryotes are more sensitive to heat, cold, and dessication, meaning they can't live in many of the harsh environments that prokaryotes are fine with. So you won't be seeing any eukaryotes colonizing the land, like prokaryotes have. Not for a very long time.

It may seem strange that these, our ancestors, don't hold a special place on planet Earth, but that's just not how evolution works. Evolution isn't striving for complexity. Rather, complexity is more like a byproduct produced as life gropes into unfilled niches.

In the long, long history of life on Earth, complexity has been punished more often than it's been rewarded. Even in modern times, in the golden age of eukaryotes, simple prokaryotic life is far more prevalent than eukaryotes. Prokayrotes may not be as flashy as eukaryotes, but you don't have to go far to find them. Take a close look at your own body, and you'll find a hundred trillion bacteria living in and on it. That's ten times the number of your own eukaryotic cells. So, even inside our own skins, we're still a niche organism.

But these eukaryotes, back here in the Proterozoic? These eukaryotes are gonna make it. They're going to survive—somehow or other—and evolve into even more specialized forms, and their descendants will in turn branch out to become every plant, animal, and fungus on modern-day Earth.

As we place them back into the water—very, very carefully, mind you—we notice strange formations dotting the bacterial mat below the surface. What the hell are those things?

No, they're not the rude artistic endeavors of the cyanobacteria, although that would fit right into the usual habits of those little bastards. These are collections of cells that seem to be forming a sort of multicellular structure. They may be prokaryote or eukaryote. We're not sure, and we can't check, because the cartoon microscope's batteries are dead.

Now, communal living isn't exactly unknown here. The bacterial mats are communities of cells. If not for those pesky eukaryote biases, we might even consider the mats a multicelluar organism. The bacteria in them do exhibit a kind of ad-hoc specialization, exchanging chemical signals and altering their habits to better support the mat. But they can't form the sort of complex structures that eukaryotic cells are uniquely capable of.

But if these formations are made of eukaryotes, and if they're purposefully growing into those complex structure, that would make them the first organized multicellular structures to appear on Earth1. In the modern era, there's hot debate on the subject. We could have settled the matter right here and now, of course, if you'd only remembered to turn the lamp off on the cartoon microscope.

Nice one.

Well, that's the most interesting thing that’s going to happen around here for a while, so maybe we should get back to that Monopoly tournament.

Okay, after four more games of Monopoly, we've wiled away 400 million years. It's now 1.9 to 1.8 billion years ago and something interesting is finally about to happen. You might want to cover your ears.

You hear that deafening shockwave, and feel that wave of superheated air that’s vaporizing your skin? Yeah, that’s the Sudbury Basin impact event. It’s a meteor that just hit Ontario, making an impact crater over a hundred and fifty miles across. That’s bigger than the crater made by the meteor that will finish off the dinosaurs in a couple billion years. The Sudbury impact rivals even the largest impact in the geological record, which occurred in what will become Africa while we were playing Monopoly. Remember the ash cloud that choked out the sun for a thousand years while you were busy buying up Baltic Avenue? Yeah, that was what that was.

So, yeah. You should definitely put some aloe on that.

This impact occurred so far back that we can't easily estimate the damage it caused to life on Earth, the way we can with later impacts. We don't know how many species it wiped out, and we're not likely to resolve the matter now, since our flesh has been incinerated and our bones buried beneath layers of ash, but I don't think I'm going out on a limb when I say this impact was nasty. It probably killed the majority of organisms on Earth, and caused the extinction of uncountable species.

So, more Monopoly?

Over our next few games, the archipelagos in the Proterozoic sea are going to drift together. The tectonic plate that we’re buried in will run into a bunch of others, and they’ll fuse together along the margins to form one giant plate called Laurentia, or the North American plate. This isn’t just normal continental drift, mind you. Continental plates bounce off each other all the time. This is the formation of a new, much bigger plate, and it will stick together until the modern era. Most of the big continental plates we have today, in fact, will be fusing together from smaller plates within the next few hundred million years. Then all those plates will come together (without permanently fusing) to form a supercontinent called variously Columbia, Nuna, or Hudsonland.

But, much like The Eagles, the supercontinent Columbia is going to split up again over our next few games of Monopoly.

Now it’s 1.2 billion years ago, and sex has just been invented.

WELL IT TOOK LONG ENOUGH, DIDN’T IT?

Well, saying sex has just been invented is a little conceited. Bacteria have been having sex for a long time, probably from the very beginning. But they don’t do it the same way we do. They just kind of shoot genes at each other, and if they’re good genes, they tend to stick around. The kind of sex that involves gametes coming together to make a third distinct organism, the kind of sex you’re most likely to regret, is what’s just been invented.

Well, actually there’s a caveat there, too. Because we don’t know if they just now invented it, but this is the earliest we’ve noticed it in the fossil record. And here it is:

Wait, I don’t think that’s right. Here are the actual pictures of the first known evidence of sex:

Wow, that’s hot. I’m surprised we didn’t have to blur those photographs of primordial sex organs.

Sex is a big deal. Without sex, even multicellular organisms can really only clone themselves, budding off little packets of cells to float away and form a genetically identical organism somewhere else. But with sex, you can blend your genes with someone else’s—preferably someone cool. No, really. That really is one of the primary advantages of sexual reproduction: you can pick someone with awesome genes, which will help carry the slack of your crappy genes. And, while these early multicelled eukaryotes couldn’t exactly pick and choose their partners—it was pretty much down to whoever’s gametes happened to float past—they at least got a partner who was a proven survivor.

Are proterozoic lifeforms ready for sex, though? Or will this entire eon turn into one big cautionary tale? We'll find out next time...

***

If you liked this article, check out the other Proterozoic and general science articles at our handy Archives.